1. Field of the Invention
The present invention relates to corrosion inhibition and more particularly to inhibition of corrosion of metal surfaces by means of a corrosion inhibitive iron carboxylate coating formed over the surface.
2. Description of Prior Art
Corrosion of metal surfaces, especially corrosion of ferrous metal surfaces, in various media has long been a troublesome problem. Such surfaces are subjected or exposed to corrosive media in a wide variety of environments. One particularly conspicuous environment prone to corrosion may be found in oil and gas wells and fields, where carbon dioxide and sulfide corrosion of ferrous metal surfaces in oil and gas wells and pipelines is especially troublesome.
One response to such corrosion problems has been the use of corrosion inhibitors that coat corrosion-prone surfaces with a film or coating. The film forms a barrier between the metal surface and the corrosive medium.
In many situations, however, conventional films have not been found to provide completely satisfactory corrosion inhibition. An especially troublesome environment has been found to be those in which the inhibitor coating is subjected to significant shear forces, such as result from high flow rates of the medium across or at the coated metal, typically ferrous metal, surface. For example, many oil wells have been producing at increasing production rates, thereby increasing the shear stress applied to the pipe walls. These shear stresses tend to wear or strip away coatings of corrosion inhibitors from the metal surface.
Another problem with conventional film-type corrosion inhibitors is that they typically are not uniformly effective in a wide variety of media. Thus, while the efficacy of some inhibitors have been found to be acceptable in water-rich media, the efficacy of such inhibitors in inhibiting corrosion has been found to be less adequate in hydrocarbon-rich media. Likewise, inhibitors useful in hydrocarbon media have been found to be less so in aqueous media. However, such inhibitors are not as effective as desired even in the medium to which they are more suited, and more effective corrosion inhibition and more persistent inhibition in both types of media and other media is desired.
As a result, compositions and techniques that form films that provide highly effective corrosion inhibition and cling more tenaciously to the metal surface, thereby to be more resistant to shear forces, are still being sought. The need for such compositions and techniques is especially significant with respect to the particularly serious corrosion problems associated with ferrous metal surfaces such as iron and steel surfaces, and with respect to corrosion caused by exposure of such surfaces to carbon dioxide and sulfides. It is also desired that such compositions and techniques be adaptable to the wide variety of media, allowing optimization for each medium, while maintaining formulatability of the inhibitor composition in an inhibitor product including in the composition and a solvent.
U.S. Pat. No. 5,385,616 discloses that certain maleated unsaturated acids react with ferrous ions in corrosion product on a metal surface to form an iron carboxylate film that adheres tenaciously to the surface, thereby affording excellent corrosion protection. However, the maleated unsaturated acids of the noted patent are of limited use in certain respects. They are generally coordinated with the medium so as not to be soluble therein, thereby to avoid breakdown of the surface film by dissolution of the film into the medium. Indeed, soluble inhibitors generally do not maintain tenacious films or even substantial corrosion resistivity, particularly when the dissolved component is removed or lost as a result of fresh flow of medium over the surface.
Therefore, if the medium is water, a water insoluble/hydrocarbon soluble inhibitor may be selected, while a water soluble/hydrocarbon insoluble inhibitor may be selected for a hydrocarbon medium. Because they are generally insoluble in the medium, they are usually applied to the surface itself, such as by painting, spraying or dipping techniques, rather than applied to the medium in which the metal surface is located.
Nevertheless, in many cases, it is difficult to design an inhibitor that is insoluble in the medium. For example, some media (such as produced fluids from wells) contain significant concentrations of both hydrocarbons and water. Moreover, it is often desirable to be able simply to add the inhibitor to the medium itself, such as by injection. Thus, selection of an insoluble corrosion inhibitor, although beneficial for film adherence, is often undesirable for convenient application. Accordingly, it is still desirable to locate corrosion inhibitors of this type that are soluble in the medium itself so that they may be applied to the medium itself, yet still maintains a tenacious film over the surface in the medium, even when the medium contains little or no dissolved inhibitor.